Lunar and Planetary Science XXXVIII (2007) 1339.pdf

THE LIGHT-TONED SEDIMENTS IN AND NEAR LOWER MAWRTH VALLIS MAY BE A DRAPE DEPOSIT. A. D. Howard1 and J. M. Moore2, 1Dept. Environmental Sciences, University of Virginia, P.O. Box 400123, Charlottesville, VA 22904-4123 ([email protected]), 2NASA Ames Research Center, M.S. 245-3, Moffett Field, CA 94035.

Introduction: Marwth Vallis and the phyllosilicate- “islands” exhibit strong layering on upper crater walls rich deposits in and near its lower terminus constitute one (Figs. 2, locations g&f). Other gently-sloping Mawrth of the most enigmatic landform assemblages on Mars. Valles walls, such as that shown at “$” in Fig.1, exhibit Marwth Vallis is generally classified as an outflow chan- discontinuous exposures of light-toned deposits, but show nel, but it lacks obvious concentrated flow sources in little evidence of eroded, exposed layers suggestive of breached basins, collapse terrain or large tributary chan- horizontal bedding. A similar situation occurs on the nels. The valley width remains a nearly constant 20 km north wall of Marwth Valles in THEMIS VIS image over most of its ~800 km length and the valley bends only V11322008. in broad curves with radii ~40 km, suggesting high vol- ume formative flows as, for example, catastrophic fluvial floods, turbidity currents, or glacial sculpture. Interbed- ded dark- and light-toned deposits occur within and on the uplands surrounding the lower end of Marwth Vallis over an elevation range from about –2500 to –3700 m. Parts of these deposits exhibit extensive and concen- trated phyllosilicate spectra [1, 2], but their origin and time of emplacement remain uncertain. Although [1, 2] suggest that the phyllosilicate minerals occur only on the plateaus surrounding Mawrth Vallis, their map (Fig. 4 in [1]) clearly shows strong phyllosilicate signatures in the floor of Mawrth Vallis in the vicinity of 341.6°E and 22.8°N including the area in Fig. 1. The prevailing inter- pretation suggests Mawrth Vallis has been excavated into an older, thick phyllosilicate-rich sequence, so that there is no necessary genetic coupling between vallis and de- posits[1-5]. We suggest an alternative interpretation in which the phyllosilicates originated as a drape deposit that post-dates, or is possibly coeval with, formation of Mawrth Vallis. Stratigraphic Relationships: Mawrth Vallis has been extensively imaged by MOC NA, THEMIS VIS and IR, and HRSC images. Despite this coverage, interpreta- tion of stratigraphic relationships is difficult because the sedimentary layering is less distinct and repetitive than in Fig. 1. Layered deposits in Mawrth Valles. Color coded other martian sedimentary deposits (e.g., [3, 6]). In exam- with blue tint in valley bottom grading to red at valley crest. ining both available MOC NA images and THEMIS VIS Deposits from lowest to highest are bright-toned layered images, we find no exposures definitively demonstrating deposits “&”, smooth-surfaced dark deposit “%”, massive that the layered deposits occurring near the floor of dark deposit “#”, bright layered deposits “@”, and dark, Mawrth Valles continue into the wallrock of the Valles. partly eroded mantle, (arrows). A dark mantle also floors Interpretation of Figs. 1-3 suggest that both the bright- the shallow tributary valley entering from the south. The toned deposits and an overlying dark mantle drape over dark mantle (arrows) appears to follow the slope of the the walls of the Valles and also comprise the layered de- valley sides. This, and the dark mantle sloping with the posits on the crater floor. Where the lower walls of tributary valley, suggests that the valley-crest dark mantle Marwth Valles have been eroded into steep slopes, dark may be equivalent to the dark valley bottom mantle “%”. grey, poorly-layered material appears to be exposed (“#” The valley crest bright deposits “@” may likewise be con- in Fig. 1). Steep exposures across a relief of about 300 m temporaneous with the similar valley-bottom deposits “&”. in the multi-channel (“braided”) section of lower Marwth By this interpretation the dark massive deposits “#” are the Valles (Figs. 2, locations a through f, and Fig. 3a,b) are materials into which Mawrth Valles was excavated, and the bright and dark deposits would have been deposited subse- partially mantled with mass wasting debris, but show little quent to valley excavation. An inverted channel capped evidence of erosional sectioning of bedded light-toned with dark deposits occurs at “C” on the bottom of Marwth deposits. By contrast, small, fresh craters impacting on Vallus. Mosaic of portions of THEMIS VIS images the valley floor and on the top surface of the inter-channel V14105004, V01873006, and V06342015 on a THEMIS IR Lunar and Planetary Science XXXVIII (2007) 1339.pdf

mosaic. Image width 30.5 km, centered at about 341.8°E and 22.7°N. Conclusions: If the light- and dark-toned sediments in the lower Mawrth Vallis region are a drape deposit, then the deposit would most likely have been deposited when the topography was not much different than present. If this is the case then a wholly fluvial origin of the de- posit is unlikely, because of the sharp drop-off to the northern lowlands at the end of the vallis. A coastal plat- form environment is a possibility, and the deposits might have derived from materials transported through Mawrth Vallis. If the layered deposits at the lower end of Mawrth Vallis post-date the formation of the valley, then deposi- tion of phyllosilicate minerals may have occurred later in martian history than suggested by [1, 2] References: [1] Bibring, J.-P. et al. (2006) Science, 312, 400-4; [2] Poulet, F. et al. (2005) Nature, 438, 623-7; [3] Edgett, K. S. (2005) Mars: Int. J. Mars Soc. Explor., 1, 5-58; [4] Farrand, W. H. et al. (2006) EOS 87(52), Abstr. P22A- 06; [5] Michalski, J. R. et al. (2006) EOS 87(52), Abstr. P22A-05; [6] Malin, M. C., Edgett, K. S. (2000) Science, 290, 1927-37.

Fig. 3. Selected portions of MOC NA images covering locations shown in Fig. 2. Images selected from MOC image M0706145. Scale bars are 500 m. Image (a) shows a steep slopes with average gradients of about 0.2. Fig. 2. Multi-channel portion of northern Mawrth Image (b) slopes at about 0.06. Note lack of horizontally Valles. Mosaic of THEMIS VIS images V11659006, bedded exposed layering. V12882008 and V13194007 on a THEMIS IR mosaic, with superimposed MOLA-derived contours (contour interval 20 m). The tops of the central “islands” are about 300-500 m above the adjacent channels floors. Image centered at 341.6°N and 24.4°N. Image width 41.5 km. Labeled boxes show locations of MOC image selections in Figure 3. La- bel (f&g) locates craters on top of “island” exposing lay- ered deposits.